Porcellanite is a siliceous duricrust which has developed within altered Cretaceous mudrocks in the Darwin city area and is well exposed in coastal cliffs to the north. Its distinctive geotechnical properties include low bulk density, variable strength that is highly dependent on moisture content, and relatively high (but inconsistent) durability. The porcellanite rock mass is inhomogeneous and anisotropic; it is unrippable at the surface, but becomes weaker with depth. It is also highly permeable in places, even karst-like, due to solution cavities. These characteristics are common to all duricrusts and result from processes of solution, replacement and redeposition by silica-laden groundwater. Such processes may have been intermittently active through most of the Cenozoic and there is evidence that they continue to the present. The upper 2–4 m of the porcellanite profile is made up of a brittle, high-strength rock with a silica content approaching that of silcrete. The underlying altered and porous siltstone is much weaker and deforms plastically under loading, due to a cellular microfabric composed largely of opaline silica replacing clay minerals. Porcellanite has long been used as the main building stone in Darwin and is now quarried for shoreline filling. Although some of it is of select fill or road sub-base quality, crushed porcellanite contains an excess of plastic fines making it unsuitable for basecourse. Porcellanite has been a failure as breakwater stone, despite producing blocks of adequate size.